Explosives detection using resonance fluorescence of bremsstrahlung radiation

ABSTRACT

A technique for detecting explosives using resonance fluorescence of bremsstrahlung radiation is disclosed. The method is particularly attractive as a way to detect bombs at airports and other transportation terminals. According to the invention, bremsstrahlung radiation is made incident on a target (e.g., a piece of luggage) to resonantly excite the atoms of the target. In one embodiment, the energies of the photons scattered directly from the target are detected and measured. These energies are characteristic of the nuclear species excited in the target, and thus the concentrations of these elements in the target can be determined. A high concentration of nitrogen and oxygen with a low concentration of carbon indicates practically without fail an explosive material. In another embodiment, the energies of photons resonantly scattered from reference scatterers composed substantially of nuclear species of interest and located downstream from the target are detected and measured. The abundance of photons of energies corresponding to nuclear species of interest detected in this embodiment is inversely related to the abundance of the species in the target.

This application is a continuation-in-part of U.S. application Ser. No.567,970, filed Aug. 15, 1990.

BACKGROUND OF THE INVENTION

This invention relates to explosives detection at, for example, airportsand other transportation terminals, and more particularly, to a methodand apparatus for detecting explosives and other materials usingresonance fluorescence of bremsstrahlung radiation.

Most explosive materials have relatively high nitrogen and oxygenconcentrations. Some common materials also have high nitrogen or highoxygen concentrations; however, almost no common materials have both thehigh nitrogen and high oxygen concentrations of explosives. Thus, thedetection of most explosives would be greatly facilitated if theabundances of nitrogen and oxygen in a sample could be determined.Practically a 100% certainty of identification could be achieved formost kinds of explosives, including plastic explosives, TNT, dynamite,ammonium nitrate, and nitroglycerin. Detection of other elements, suchas chlorine, could further decrease the uncertainty of identificationfor an even wider range of explosives.

There are many requirements that bomb detecting apparatus at airportsmust meet. First, the measurements must be reliable. Also, the searchesmust be non-invasive and non-destructive. Since the articles to beexamined are sizeable, the use of penetrating radiation is attractive;however, the radiation must not leave the baggage radioactive. Radiationshould be easy to shield so as to make the environment safe for peoplewithout the need for bulky and expensive walls. The capability to imagea target is often important in bomb detection. In addition, measurementsshould not take more than several seconds per piece of baggage. Finally,as there is a need for thousands of these facilities, with several atmost airports, cost is an important consideration.

SUMMARY OF THE INVENTION

The method of the present invention exploits the resonant scattering ofphotons by nuclei. It involves resonantly exciting the nuclei of atarget, a suitcase for example, with a bremsstrahlung photon beamincident on the target. In one embodiment, the subject of copending U.S.application Ser. No. 567,970, the energies of the photons scattereddirectly from the target are measured. The energies of the scatteredphotons are characteristic of the spacings between the quantizied energystate of each nuclear species comprising the target. For example, oxygenhas a discrete energy level at 6.92 MeV of excitation characterized byeven parity and two units of angular momentum. A bremsstrahlung beamincident on a target with oxygen will excite some of the nuclei to thisstate. The state will subsequently decay with a lifetime of about6.8×10⁻¹⁵ seconds by emitting a photon with an energy of 6.92 Mev.

Apparatus according to this embodiment of the present invention includesa source of bremsstrahlung radiation comprising an electron sourceproducing electrons incident on a bremsstrahlung target to producebremsstrahlung radiation, a beam stopper to absorb the electrons, afilter to absorb the low energy end of the bremsstrahlung spectrum, andan aperture to collimate the bremsstrahlung radiation. Appropriateshielding surrounds the bremsstrahlung source. The apparatus furtherincludes detecting apparatus to capture, measure, count, and record theenergies of photons scattered from a target. This detecting apparatusalso employs appropriate filtering and shielding. A beam dump absorbsthe photons transmitted through the target. Shielding surrounds theentire set-up to protect the public from photons and neutrons. Acomputing apparatus accepts signals from the detecting apparatus anduses this data to distinguish a "suspicious" suitcase from a "normal"one.

The angular distribution of the scattered photons is very broad, and,for the purposes of qualitatively describing the present application,may be considered almost isotropic. Therefore, detectors at almost anyangle will detect the scattered photons. The detected intensity,normalized by the incident beam intensity and detector efficiency, willyield an accurate measure of the abundances of various elements. Themeasurement can be achieved in several seconds with a reasonably simpleset of analysis algorithms.

The bremsstrahlung beam can be collimated to a small spot or a thinstrip, for example. By sweeping the collimated beam, one can image thetarget. Imaging can also be achieved by flooding the target withbremsstrahlung radiation and using directional detectors. A combinationof the two techniques is also possible.

An alternate detecting scheme is disclosed in the present applicationwhich involves making the photons that are transmitted through thetarget also be incident on one or more reference resonance scattererslocated downstream from the target. Each reference scatterer is composedsubstantially of one or more nuclear species of interest and isassociated with a detecting apparatus to detect photons scattered fromthe reference scatterer. If the target contains an abundance of anuclear species of interest, photons of energies corresponding to thatnuclear species will be resonantly absorbed and will not become incidenton the reference scatterers. Thus the signal recorded by the detectingapparatus associated with a reference scatterer comprising that nuclearspecies will be diminished when the target is placed in the path of thebremsstrahlung beam. This detecting scheme is compatible with, and maybe used in combination with, detecting the photons scattered directlyfrom the target.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of one embodiment of the explosivesdetector of the present invention.

FIG. 2 is a schematic drawing of one embodiment of a detector arrayadapted for directional detection.

FIG. 3 is a schematic diagram of one embodiment of an alternatedetecting scheme of the present invention.

FIG. 4 is a schematic drawing of apparatus for conveying suitcasesthrough the explosives detector.

DESCRIPTION OF THE PREFERRED EMBODIMENT

One embodiment of the present invention which was disclosed in copendingU.S. application Ser. No. 567,970, and is particularly appropriate fordetecting explosives in suitcases, is illustrated schematically inFIG. 1. As shown, an electron source 10 provides a beam of electrons 12incident on a bremsstrahlung target 14 to generate a bremsstrahlungphoton beam 16. The bremsstrahlung target 14 is preferably followed by abeam stopper 18 to stop the electrons 12. A filter 20 preferably followsthe beam stopper 18 to filter out low energy photons from thebremsstrahlung beam 16. An aperture 22 is employed to collimate thebremsstrahlung beam 16. Shielding 24 encloses thebremsstrahlung-generating apparatus 5. A target 26 (a piece of luggage,for example) is placed in the path of the bremsstrahlung beam 16 (by aconveyor belt, for example). The incident beam 16 resonantly excites theatoms of the target, and photons 28 are scattered from the target 26.The energies of the scattered photons 28 are characteristic of thespacings between the quantized energy states of the nuclei of the target26. Detecting apparatus 30, including an array of detector 32, captures,measures, counts, and records the energies of the photons 28 scatteredin a given direction or directions. The detecting apparatus 30preferably further includes a filter 34 over the face of each detectorto absorb low energy photons, and shielding 36 and 38. As scattering anddiffraction from the collimating aperture 22 could lead to a significantamount of photons directed toward the detecting apparatus 30, a shadowshield 40 between the aperture 22 and the detecting apparatus 30 issuggested. A beam dump 42 is provided to absorb the energy of the beam16 not absorbed in the target 26. Shielding 62 encloses the entiredevice while allowing convenient means for the entry and exit oftargets. Data from the detecting apparatus 30 is sent to a computingapparatus 44 which analyzes the data and determines the abundances ofparticular elements. The computing apparatus is preferably adapted tocompare the data for each piece of luggage to profiles of "normal"luggage to determine if a piece of luggage should be considered"suspicious".

The electron source 10 is any accelerator capable of producing a beam ofelectrons 12 at the required energies and with the required intensityand duty ratio. Among the suitable accelerators are linear accelerators,electrostatic accelerators, microtrons, and betatrons, all of which arecommercially available. The electron energies required in the presentinvention are on the order of 10 MeV. It is preferable that energies notexceed about 10 MeV so that radioactivity and neutron production doesnot occur. The beam intensity required is on the order of at least 10μA; however, an accelerator capable of producing a beam of intensity inexcess of 100 μA is preferred. Duty ratio is important and more than onepercent is very desireable. Cost and size are also importantconsiderations.

The bremsstrahlung target 14 is a suitable thickness of any materialwith a high atomic number of nuclear charge (high Z) and a high meltingand boiling point, such as tungsten, tantalum, thorium, or uranium. Asan electron in the beam passes through the bremsstrahlung target, itproduces the electromagnetic radiation called bremsstrahlung radiationwhich consists of quanta of photons. The spectrum of energies of thesephotons is continuous, spanning from very low energies to a maximumenergy equal to the kinetic energy of the electrons in the beam. Thisradiation goes by various terms depending on the target and filteringprocess employed. The terms include thin target bremsstrahlung, thicktarget bremsstrahlung, filtered bremsstrahlung, and others. Here, theterm bremsstrahlung is used to encompass all varieties of bremsstrahlungradiation.

A bremsstrahlung target comprising a thickness of tungsten about theorder of 1 gram/cm² is appropriate for most embodiments of the targetinvention. A substantially thinner target would not provide a sufficientintensity of high energy photons. A substantially thicker target wouldbe self-absorptive of the high energy photons.

The half-angle θ_(n) of the natural angular spread of the bremsstrahlungbeam for target thickness approaching zero is θ_(n) ≈m_(o) C² /E_(o),where θ is in radians and is measured relative to the direction of theincident electron beam, m_(o) is the mass of the electrons, c is thespeed of light, and E_(o) is the energy of the electrons. For 10 MeVelectrons, θ_(n) ≈2.9°. This natural collimation is not completelyuseful in the present application, however, since the intensity ofphotons produced with a target thin enough to provide this naturalcollimation does not meet the present requirements without the use ofmuch higher electron beam currents. An increased current would require aconsiderably more expensive accelerator and shielding apparatus.

Energy is also absorbed from the electrons in the beam by atomicionization and by the excitation of the atomic electrons in thebremsstrahlung target. In the process of interacting with the atoms ofthe target, the electrons often suffer a deflection. The cumulativeeffect of these deflections is to increase the angular spread of thebremsstrahlung beam. The rms half-angle θ of the angular spread can beestimated as ##EQU1## where θ is in radians, P is the momentum of theelectrons in MeV/c,β is the velocity of the electrons in units of thespeed of light (and can be taken to be about 1 for this example), and Tis the thickness of the bremsstrahlung target in radiation lengths thatthe electrons have traversed. As an example, 1.0 gram/cm² of tungsten isabout 0.15 radiation lengths and θ is therefore about 0.77 radians or 45degrees for a 10 MeV electron beam. Small angles of natural angularspread are therefore only achieved when a very thin bremsstrahlungtarget is used and overall efficiency for converting the electron energyinto photons is comprised. Collimating the radiation with an apertureallows almost any beam shape and angular size approaching that producedby the scattering.

The beam stopper 18 follows the bremsstrahlung target. The beam stopperis preferably a low Z material, such as carbon, of a thicknesssufficient to absorb the remaining energy of the electrons 12 from theelectron source 10. This low Z material will not generate bremsstrahlungradiation with the same efficiency as the bremsstrahlung target 14, butwill be heavily ionizing and therefore will stop the electrons. Thethickness of carbon required to stop substantially all the electrons ina 10 MeV beam is about 5.5 gram/cm².

It is not advised to simply extend the thickness of the bremsstrahlungtarget 14 to act as the stopper, because a substantial increase in itsthickness would be required to perform this function and its high Zwould attenuate more of the useful high energy photons than the stopper18 of the lighter material. For example, a thickness of 8 gram/cm² oftungsten would be required to stop substantially all electrons of a 10MeV beam.

It is possible to deflect the electron beam with the use of magnets sothat the stopper 18 need not be placed in the line of the bremsstrahlungbeam 16. However, since a stopper of low Z material will notsubstantially affect the bremsstrahlung radiation at the energy levelsof interest, this scheme introduces unnecessary complication andexpense. On the other hand, the use of a magnet to deflect the electronbeam by modest amounts before its strikes the bremsstrahlung target, inorder to sweep the resultant direction of the bremsstrahlung beamproduced by the bremsstrahlung target, is a possible and usefulembodiment for the purposes of imaging.

A filter 20 preferably follows the beam stopper 18. The filter ispreferably a low Z material of a thickness sufficient to absorb the lowenergy end of the bremsstrahlung spectrum preferentially over the highenergy end, where the nuclear states of interest lie. The low energyphotons in most embodiments of the present invention are those in theregion of about 2 MeV or less. A suitable material for this filter is amaterial with atomic number in the range of carbon to iron. The filtermay also be made using a combination of suitable materials. The filtercan be tuned to optimize its filtering performance over a desired energyrange by selecting the atomic number of the filter material or materialsand the thickness of the filter.

An indicated above, it is not practical to rely only on the naturalcollimation of the bremsstrahlung radiation, since beam intensity issacrificed by the use of a very thin bremsstrahlung target. Therefore,the aperture 22 is required to produce a collimated beam. The apertureis preferably graded to result in a well-defined beam with little halo.It is advantageous to make the aperture rapidly adjustable to permitquick changes in the beam collimation. Preferred aperture geometries arediscussed below in conjunction with imaging techniques.

The shielding 24 includes a first layer 46 of a high Z material, such asbismuth, lead, or iron, of sufficient thickness to absorb substantiallyall of the energy of the electrons and of the photons.

The first layer of shielding 46 and the collimating aperture 22 could bea prolific source of neutrons from the so-called Giant Electric Dipoleresonance in heavier nuclei. These neutrons must be shielded from thedetectors so as not to produce too much background from neutron inducedreactions. A second layer of shielding 48 is therefore required tosubstantially absorb the neutrons produced by the bremsstrahlungradiation. This second layer is preferably a hydrogenous material loadedwith boron or lithium to preferentially capture the neutrons so that nohigh energy photons are emitted. Even at 10 MeV, substantial neutronshielding is required. At higher energies, this problem will be muchmore serious. This loaded hydrogenous material might also be required tocover the exit port of the collimating aperture.

A final layer of shielding 50 of a high Z material such as bismuth,lead, or iron, is required to capture photons generated by neutroncapture in the second layer 48 and the outer regions of the first layer46.

The cross section for the absorption of the photons of thebremsstrahlung beam 16 by the nuclei of the target 26 with ground statespin equal to zero, assuming only photon decay to the ground state (andnot proton and other kinds of decay) is involved, is given by ##EQU2##where λ is the photon wavelength, ε is the photon energy, ε_(o) is theenergy of the state, Γ is the full width of the state, and G is astatistical factor equal to 2l+1. In this last expression, l is themultipolarity of the transition. For nuclei with ground states of spinequal to zero, l is also the spin of the excited state. This crosssection has the usual resonance behavior. The peak value of this crosssection occurs when ε=ε_(o) and has the value λ² (2l+1)/π. Theintegrated cross section which is of interest for counting rateestimates is given by σ=λ² Γ(2l+1)/π; that is, the peak value of thecross section multiplied by the width Γ. If other modes of decay arepossible, the ground state strength is attenuated. If the ground statespin is different from zero, the formula, although not precise, is stilluseful for estimating the approximate cross sections.

The nuclear states of interest in the present invention are states thathave a width Γ that is mostly due to photon decay. Cascades viaintermediate states that produce photon energies that are sufficientlyhigh so that they can be detected above background are acceptable. Thewidths of these states of interest are very small. For the decay of the6.92 MeV state in oxygen to the ground state, Γis on the order of 1/10eV. However, the cross sections are large in the peak because λ islarge. A value of about 200 barns is expected for the 6.92 MeV state inoxygen. This means that most of the photons in the bremsstrahlungspectrum that lie within the resonance peak would be absorbed by athickness of material that has about 1/10 of a gram of oxygen per cm².The same would be true for nitrogen and carbon, or any other material ofinterest.

It is important to note that the line widths are made wider by thethermal motion of the molecules, and the technique will not suffer anysignificant loss of penetration depth because of the large peak crosssections. Unlike the situation where one is scattering a narrow line ofphotons from a resonance, the thermal broadening of the peak does notcause a loss of sensitivity. In the bremsstrahlung spectrum there arealways photons to scatter at whatever energy the resonance is shifted toby thermal molecular or atomic motion.

Nitrogen states of particular relevance to the preferred embodiment ofthe present invention include:

the 4.92 MeV 0⁻ state which decays to the 1⁻ ground state with a widthof 0.084 eV.

the 7.03 MeV 2⁺ state which decays to the 1⁺ ground state with a widthof 0.078 eV.

Oxygen states of particular relevance to the preferred embodiment of thepresent invention include:

the 6.92 MeV 2⁺ state which decays to the 0³⁰ ground state with a widthof 0.097 eV.

the 7.12 MeV 1⁻ state which decays to the 0⁺ ground state with a widthof 0.055 eV.

A carbon state of particular relevance to the preferred embodiment ofthe present invention is:

the 4.44 MeV 2⁺ state which decays to the 0⁺ ground state with a widthof 0.011 eV.

Another oxygen state of interest in the 12.53 MeV 2⁻ state which: decaysto a 6.13 MeV 3⁻ state producing a 6.40 MeV photon and a 6.13 MeV photonwith a width of 2.1 eV; decays to a 7.12 MeV 1⁻ state producing a 5.41MeV photon and a 7.12 MeV photon with a width of 0.5 eV; and decays to a8.87 MeV 2⁻ state producing a 3.66 MeV photon, along with a 1.74 MeVphoton and a 6.13 MeV photon in cascade most of the time, with a widthof 0.9 eV. Another carbon state of interest is the 12.71 MeV 1⁺ statewhich: decays to the 0⁺ ground state with a width of 0.35 eV; and decaysto the 4.44 MeV 2⁺ state producing a 8.27 MeV photon and a 4.44 MeVphoton with a width of 0.053 eV. The total widths of the 12.53 MeV statein oxygen and the 12.71 MeV state in carbon are 97 eV and 18 Evrespectively. These states decay preferentially by proton and/or alphaemission and the photon intensities will be considerably reduced. Thus,these states are not a part of a preferred embodiment because of theirlow intensity and high energy.

The 15.11 MeV 1⁺ carbon state, which decays to the 0⁺ ground state witha width of 38 eV, is of great interest. Its total width is about 43.6 eVand most of the width is ground state decay. However, exciting thisstate requires an electron beam with energies significantly above theapproximate limit of 10 MeV specified above to limit the production ofradioactivity and neutrons. Thus, the detection of carbon via thisstate, while attractive because of the very strong scattering, wouldinvolve additional complication and expense.

There are numerous states of the isotopes chlorine 35 and chlorine 37that are of interest, resulting in photons with energies up to about 6MeV with radiative widths in the range of the previous considerations.Thus, chlorine abundances can be determined as easily as those ofoxygen, nitrogen, and carbon.

Many other elements have states of the appropriate energy and radiativewidths. Therefore, the present invention is general in its usefulness.The method of the present invention can be used to detect almost anyelement of interest in a target. In addition to the explosives detectionembodiment here, apparatus according to the present invention could beused in many industrial and commercial applications.

An important consideration for the estimation of count rates is thenumber of photons that are found within the width of the resonance inthe bremsstrahlung spectrum. For a 10 MeV electron accelerator producing10 μA of electrons, the bremsstrahlung spectrum can have about 10⁶photons per eV at 7 MeV. Thus, for a state that is about 1/10 eV wide,about 10⁵ photons will lie within the range of the resonance and will beabsorbed from the beam and re-emitted over all angles with very roughlyan isotropic distribution. If 1/10 of the sphere is subtended withdetectors that are 10% efficient, the counting rate will be about 1000per second.

In this example, it was assumed that all the resonant photons areabsorbed by the target 26. This is roughly correct for a small amount ofmaterial because of the large cross sections when thermal motion isneglected. With thermal motion there are always more photons availableto scatter and thus the counting rates could be larger. However, theexact result depends on the geometry and thickness of the explosivematerial.

It is important to compare the signal with the expected background. Inthe preferred embodiments, the signal is a set of photo lines withenergies of about 3-8 MeV. The background comes from the scattering ofphotons out of the beam by atomic processes. The major process isCompton scattering. Fortunately, if detectors are placed at scatteringangles larger than 90 degrees to the bremsstrahlung beam, these Comptonscattered photons will have energies below 0.5 MeV. The greater theangle past 90 degrees, the lower the energies of the Compton scatteredphotons, to a limit of 0.25 MeV for backscatter at 180 degrees. Pairproduction will also provide secondary bremsstrahlung that ispredominantly forward peaked, as well as 0.5 MeV photons from e⁺ e⁻annihilation. There are coherent processes like Rayleigh scattering, butthese should be very small at back angles, in particular for energies ofa few MeV and above. Thus, there is no background in the energy regionabove 0.5 MeV except from multiple scattering processes and pile-up inthe counters.

In order to minimize the effects of Compton scattering and maximize thesignal-to-noise ration, the detecting apparatus 30 should be placed atan angle φ with respect to the bremsstrahlung beam 16 of considerablymore than 90 degrees. An angle approaching 150 degrees is stronglypreferred to make the signal stand out sufficiently from the noise ofmultiple processes.

There are many ways to arrange individual detectors 32 within adetecting apparatus 30. A preferred geometry is an annular array ofdetectors positioned to collect photons scattered back from the targetat an angle φ of about 150 degrees. This is the embodiment illustratedin FIG. 1 where the detecting apparatus 30 is shown in cross-section. Ofcourse, the array need not form a complete annulus around the path ofthe bremsstrahlung beam. For some embodiments, a vertical bank ofdetectors, one on each side of the bremsstrahlung beam 16, is preferred.

There are a variety of suitable detectors 32 that can be used in thedetecting apparatus 30. In one embodiment, the detectors are intrinsicgermanium ionization chambers where charge detectors pick up theelectrical signal. In another embodiment, the detectors arescintillators such as NaI and BGO. The light emitted by thesescintillators in response to incident photons can be converted to anelectrical signal using a photo multiplier tube. This can be almost 100%efficient, leading to higher counting rates in the above example. It isimportant to recognize that the circuitry of the detecting apparatusmust accommodate high counting rates and reduce pile-up.

The filter 34 is preferably placed in front of the detectors 32 tofilter out low energy photons while passing the high energy photons ofinterest. A suitable material for this filter is a material with a low Zsomewhere between that of carbon and iron. A high Z shield 36 ispreferably placed around all but the face of the detector apparatus toabsorb photons incident on the back and sides of the apparatus. Lead,bismuth, and iron are appropriate materials for this purpose. It may benecessary to include a shield 38 of hydrogenous material loaded withboron or lithium around the detectors to shield against neutronsgenerated in the luggage and beam dump. The high Z shadow shield 40 ispreferably placed in the path between the aperture and the detectors toabsorb all high-energy photon scattered from the aperture in thedirection of the detectors. Lead is a suitable material for thispurpose. The function of the shadow shield 40 could be performed by aheavy section of the shield 36.

The beam 16 passing through the target 26 will be attenuated by arelatively small amount. This beam must be absorbed in a beam dump 42designed to absorb substantially all of the energy. A suitable beam dumpfor 10 MeV may be a layer 52 of a hydrogenous material containing boronor lithium, a layer 54 of carbon, and a layer 56 of iron in a very deepcavity formed in a shield 58 of lead or iron, for example, to shield thesides and the detectors from back-streaming low energy photons. A layer60 of a hydrogenous material containing boron or lithium preferablysurrounds the outside of this shield. The depth of this cavity, the beamdimensions, the directive collimation of the detectors, and the exactlocation of the detectors are related parameters that must be madecompatible so as to not allow back-scattered photons from the beam dumpto enter the detectors. Additional shadow shields may be set up to helpmeet this goal.

Imaging can be achieved in a variety of ways with the technique of thepresent invention. The luggage can be scanned with the beam by movingthe bremsstrahlung-generating apparatus 5, the target 26, or simply theaperture 22. The electron beam can also be deflected by a magnet tosweep the bremsstrahlung beam direction. Preferred beam geometries arespots and stripes.

For example, if the beam 16 is collimated using a small square apertureto an average angle of Ω≈1/20 radians (about 3 degrees), the spot 1meter from the aperture will be about 10 cm×10 cm, an excellent size forimaging the contents of a piece of luggage. If each measurement takesabout 1/4 second, an entire suitcase could be scanned in severalseconds.

If the beam 16 is collimated using a vertical slit aperture to produce athin stripe of 10 cm width at the point of incidence with a piece ofluggage, a 60 cm long suitcase could be scanned in a few seconds as thesuitcase moves on a conveyor belt. Alternatively, the beam 16 could becollimated into a spot swept vertically by an adjustable collimator orby magnetic deflection of the beam. Even if the collimation is in theform of a vertical stripe, the central intensity remains the highest,reflecting the natural collimation, and magnetic deflection of the beamwill be useful for imaging.

In another technique, a large portion of the suitcase is flooded withbremsstrahlung radiation by using a large aperture, and the detectors 32are adapted to be direction-specific, as illustrated in FIG. 2. In thisembodiment, part of the filter 34 covering the faces of the individualdetectors 32 is replaced with high Z shielding 36. A column of low Zfilter 34 remains for each individual detector. In this way, eachdetector can be designed to only detect photons scattered from a smallspecific region of the suitcase in a particular direction. An array ofsuch detectors can easily be designed to image the entire suitcase to adesired degree of resolution.

A combination of the above imaging techniques results in a furtherembodiment of the present invention. For example, a thin slit aperturecould be used to radiate thin vertical strips of the suitcase as thesuitcase moves on a conveyor belt. The width of the strip will determinethe horizontal resolution of the imaging. The vertical resolution couldbe increased by using directional detectors. Such a method would resultin fast measurements at a high resolution.

Use of a rapidly adjustable collimating aperture 22 results in furtherembodiments with important advantages. For example, a piece of luggagecould first be flooded with bremsstrahlung radiation in an effort todetect explosives in the form of thin sheets and/or to obtain an initialestimate of the abundances of various elements in its contents. Theaperture could then be stopped down to image the suitcase in an effortto detect more localized explosive materials.

Another mode of operation is possible with the apparatus of the presentinvention. With a second bank of detectors located behind the target 26to detect photons transmitted through the target, the intensity ofphotons absorbed in the target can be monitored. In this way, a veryprecise image of the transmission density of the target can beconstructed. Such an image will identify specific areas of high materialdensity in the target which would be a further aid in detectingexplosive materials. Similar density imaging could be achieved bydetecting the low energy back-scatter from the target.

Shielding 62 is required to protect the public from photons and neutronsgenerated by the explosives detection device. This shielding shouldenclose the device, while allowing a target to be quickly andconveniently moved into and out of the device. For example, asillustrated in FIG. 4, if the targets are suitcases on a conveyor belt70, two sets of double doors 72 and 74, one set on each side of thedevice, would permit the entry and exit of the suitcases. This method isused in other explosives detection devices. Since the electronaccelerator can be rapidly turned off and on, additional safety isprovided by switching the accelerator off while targets are enteringand/or exiting the device.

The fact that the electron accelerator can be rapidly switched on andoff can be used to other advantages. For example, when the target is alarge container, ranging could be achieved by making pulses ofbremsstrahlung radiation incident on the target and measuring the delayof the photons scattered to the detectors. In this way, the depth of anexplosive material into the container could be approximated. Pulses ofless than a nanosecond in width are possible. The beam from anaccelerator is often bunched, or can be made to be bunched, insubnanosecond intervals.

The computing apparatus 44 is adapted to analyze the data obtained bythe detecting apparatus 30. As with other explosives detecting devices,profiles of elements, such as nitrogen and oxygen, as they appear in"normal" suitcases are preferably either modelled or experimentallydetermined. A suitcase which deviates significantly from these profileswould be considered "suspicious". The computing apparatus can be easilyadapted to compare data to stored profiles. If the profiles arerigorously determined, a high probability of explosives detectionaccompanied by a low rate of false alarms will be achieved.

An alternate detecting scheme according to the present invention isillustrated in one embodiment in FIG. 3. For simplicity, only thoseelements relevant to the detecting scheme are shown. As shown, thebremsstrahlung beam 16 is made incident on the target 26. As the beam 16passes through the target 26, photons will be resonantly absorbed by thenuclei of the target. The energies of the absorbed photons correspond tothe spacings between the quantized energy states of each nuclear speciesin the target. For these specific energies, the transmitted beam will bedepleted of photons. For example, if the target contains nitrogen,photons of energies corresponding to nitrogen will be selectivelyabsorbed. The amount of photons absorbed depends on the quantity ofnitrogen in the target. Thus, the intensities of the photons of specificenergies transmitted through the target contains information about thenuclear composition of the target. This information is exploited in theembodiment shown in FIG. 3. As shown, a series of reference resonancescatterers 64 are arranged behind the target 26. Each referencescatterer is composed of one or more of the elements that the explosivesdetecting device is to detect. A series of detecting apparatuses 66 isadapted to capture, measure, count, and record the photons 68 resonantlyscattered from each of the reference scatterers 64. For example, in asimple embodiment, two reference scatterers are provided, one ofnitrogen, the other of oxygen. A detecting apparatus is adapted todetect photons resonantly scattered from the nuclei in the nitrogenscatterer and another detecting apparatus is adapted to detect photonsresonantly scattered from the nuclei in the oxygen scatterer.

This detecting scheme operates as follows. If no target 26 is placed inthe path of the beam 16, the beam will directly strike the first of thereference resonance scatterers 64. The detecting apparatus associatedwith the first reference scatterer will detect a large amount of photonscorresponding to the nuclear species of the first reference scatterer.If a target 26 with a relatively small amount of the nuclear species ofthe first reference scatterer is placed in the path of the beam, thisstrong signal at the first detecting apparatus will be diminished byonly a relatively small amount. If however, a target 26 with arelatively large amount of the nuclear species of the first referencescatterer is placed in the path of the beam, this signal will bediminished considerably, due to the resonant absorption of the photonsof energies corresponding to the nuclear species of interest in thetarget 26. Thus, an abundance of a nuclear species of interest in atarget 26 will be detected as a decrease in the signal from thedetecting apparatus associated with a reference scatterer composedsubstantially of that nuclear species. Photons of energies notcorresponding to the nuclear species of which a reference scatterer issubstantially composed will be attenuated by only a relatively smallamount. Thus, the method of detecting the nuclear species of the firstreference scatterer extends to the each subsequent reference scatterer.An advantage of this detecting scheme is that if the energiescorresponding to two or more nuclear species of interest are very close,the detecting apparatus of the embodiment of FIG. 1 may have difficultydistinguishing the contributions from the two or more nuclear species.However, in the embodiment of FIG. 3, the energies corresponding to eachnuclear species are detected separately, this ambiguity is diminishedconsiderably, and the ability of the detecting apparatus to resolveclosely spaced photon energies is no longer very important. When theenergies corresponding to two or more nuclear species do not interfere,a single reference scatterer can be composed of a combination of thespecies.

The requirements for each detecting apparatus 66 in the series aresimilar to those described for the detecting apparatus 30 of theembodiment of FIG. 1. Thus, each detecting apparatus preferably includesan array of detectors, and shielding and filtering means as indicated byelements 32, 34, 36, and 38 of FIG. 1. The various detecting apparatusconfigurations discussed above are applicable to the embodiment of FIG.3. In order to minimize the effects of Compton scattering and maximizethe signal-to-noise ratio, the detecting apparatuses 66 should be placedat an angle φ with respect to the bremsstrahlung beam 16 of considerablymore than 90 degrees. An angle approaching 150 degrees is stronglypreferred to make the signal stand out sufficiently from the noise ofmultiple processes. It must be noted that backscatter from any of thereference scatterers 64 and the detecting apparatuses 66 must be takeninto account when positioning the reference scatterers and detectingapparatuses. Appropriate shielding may be required to isolate thedetectors from backscatter from reference resonance scatterers withwhich they are not associated. The adaptions for direction detectionpictured in FIG. 2 are also appropriate for this alternate detectingscheme. Similarly, the imaging methods described above can be employedin this scheme. A further imaging scheme appropriate to this embodimentemploys reference resonance scatterers which are small with respect tothe dimensions of the target 26. Imaging can then be achieved while thetarget 26 is flooded with bremsstrahlung radiation by moving thescatterers to scan the extent of the target.

All elements not shown in FIG. 3, such as the bremsstrahlung source, thebeam dump, the shielding, and the computing apparatus are similar tothat shown in FIG. 1. In this alternate detecting scheme, the computingapparatus is adapted to compare the intensities of photon energy levelsdetected by the detecting apparatuses 66 to reference values. Clearly anappropriate set of reference values are the intensities measured whenthe target 26 is not placed in the path of the bremsstrahlung beam. Inthat way, the decreases in intensity measured when the target is placedin the path of the beam can be directly related to the composition ofthe target. Alternatively the reference values may be determined fromprofiles of "normal" targets, as discussed above in relation to thefirst detecting scheme.

A further detecting scheme involves a combination of the detectingschemes illustrated in FIG. 1 and FIG. 3. It may be advantageous todetect both the photons resonantly scattered directly from the target 26and those resonantly scattered from the reference scatters 64. The twoschemes are completely compatible.

It is recognized that variations and modifications of the presentinvention will occur to those skilled in the art, and it is intendedthat all such variations and modifications be within the scope of theclaims.

What is claimed is:
 1. A method for detecting explosive materials in atarget, the explosive materials containing characteristically large orsmall amounts of one or more nuclear species of interest,comprising:resonantly exciting nuclei of the target with a beam ofbremsstrahlung radiation, resonantly exciting nuclei of one or morereference scatterers with the beam of bremsstrahlung radiationtransmitted through the target, each said reference scatterer comprisingone or more of the nuclear species of interest, measuring the intensityof photons at energies of interest scattered from each said referencescatterer in a direction or directions, said energies of interest foreach reference scatterer corresponding to the spacings between thequantized energy states of the nuclear species of interest of which thereference scatterer is comprised, and estimating the abundance of eachnuclear species of interest in the target from the measured intensity ofphotons scattered from the reference scatterer or scatterers comprisingthe nuclear species.
 2. A method for detecting explosive materials in atarget, the explosive materials containing characteristically large orsmall amounts of one or more nuclear species of interest,comprising:resonantly exciting the nuclei of the target withbremsstrahlung radiation, resonantly exciting nuclei of one or morereference scatterers with the beam of bremsstrahlung radiationtransmitted through the target, each said reference scatterer comprisingone or more of the nuclear species of interest, measuring the intensityof photons at energies of interest scattered from each said referencescatterer in a direction or directions, said energies of interest foreach reference scatterer corresponding to the spacings between thequantized energy states of the nuclear species of interest of which thereference scatterer is comprised, and comparing the intensities ofphotons at energies of interest to profiles of intensities forcomparable targets not containing explosives and/or to comparabletargets containing explosives to determine if the target is likely tocontain explosives.
 3. A method for detecting one or more nuclearspecies of interest in a target comprising:resonantly exciting thenuclei of the target with bremsstrahlung radiation, resonantly excitingnuclei of one or more reference scatterers with the beam ofbremsstrahlung radiation transmitted through the target, each saidreference scatterer comprising one or more of the nuclear species ofinterest, and detecting photons at energies of interest scattered fromeach said reference scatterer in a direction or directions, saidenergies of interest for each reference scatterer corresponding to thespacings between the quantized energy states of the nuclear species ofinterest of which the reference scatterer is comprised, whereby thedetection of an insufficient intensity of photons corresponding to thespacings between the quantized energy states of a nuclear species ofinterest indicates the presence of at least a threshold amount of thenuclear species in the target.
 4. A method for detecting one or morenuclear species in a target comprising:resonantly exciting the nuclei ofthe target with bremsstrahlung radiation, resonantly exciting nuclei ofone or more reference scatterers with the beam of bremsstrahlungradiation transmitted through the target, each said reference scatterercomprising one or more of the nuclear species of interest, measuringphotons at energies of interest scattered from each said referencescatterer in a direction or directions, said energies of interest foreach reference scatterer corresponding to the spacings between thequantized energy states of the nuclear species of interest of which saidreference scatterer is comprised, and estimating the abundance of eachnuclear species of interest in the target from the measured intensity ofphotons scattered from the reference scatterer or scatterers comprisingthe nuclear species.
 5. The method of claim 1 furthercomprising:measuring the intensity of photons at energies of interestscattered from the target in a direction or directions, said energies ofinterest corresponding to the spacings between the quantized energystates of nuclear species of interest, said nuclear species of interestbeing the same as of different from the nuclear species referred to inclaim 1, and estimating the abundance of nuclear species of interest inthe target from the measured intensity of photons scattered from thetarget which correspond to nuclear species of interest.
 6. The method ofclaim 2 further comprising:measuring the intensities of photons atenergies of interest scattered from the target in a direction ordirections, said energies of interest corresponding to the spacingsbetween the quantized energy states of nuclear species of interest, saidnuclear species of interest being the same as or different from thenuclear species referred to in claim 2, and comparing the intensities ofthe photons at energies of interest scattered from the target toprofiles of intensities for comparable targets not containing explosivesand/or to comparable targets containing explosives to determine if thetarget is likely to contain explosives.
 7. The method of claim 3 furthercomprising:detecting photons at energies of interest scattered from thetarget in a direction or directions, said energies of interestcorresponding to the spacings between the quantized energy states ofnuclear species of interest, said nuclear species of interest being thesame or different nuclear species referred to in claim 3, whereby thedetection of a sufficient intensity of photons scattered from the targetcorresponding to the spacings between the quantized energy states of anuclear species of interest indicates the presence of at least athreshold amount of that nuclear species in the target.
 8. The method ofclaim 4 further comprising:detecting photons at energies of interestscattered from the target in a direction or directions, said energies ofinterest corresponding to the spacings between the quantized energystates of nuclear species of interest, said nuclear species of interestbeing the same or different nuclear species referred to in claim 4, andestimating the abundance of nuclear species of interest in the targetfrom the measured intensity of photons scattered from the target whichcorresponds to nuclear species of interest.
 9. The method of claims 1,2, 3, 4, 5, 6, 7, or 8 wherein the target is imaged by collimating saidradiation to a beam, the beam cross-section having at least onedimension which is small with respect to the target, and scanning saidbeam over the extent of the target by moving the beam, the target, orboth.
 10. The method of claims 1, 2, 3, 4, 5, 6, 7, or 8 wherein thetarget is imaged by collimating said radiation to a beam having across-section which is on the order of the size of the target, floodingthe target with said radiation, and employing directional detection ofthe scattered photons.
 11. The method of claims 1, 2, 3, 4, 5, 6, 7, or8 wherein the target is imaged by a combination of the methodsof:collimating said radiation to a beam, the beam cross-section havingat least one dimension which is small with respect to the target, andscanning said beam of over the extent of the target by moving the beam,the target, or both, and employing directional detection of thescattered photons.
 12. The method of claims 1, 2, 3, 4, 5, 6, 7, or 8wherein the target is imaged by employing reference scatterers which aresmall with respect to the dimensions of the target, by collimating saidradiation to a beam having a cross-section which is on the order of thesize of the target, flooding the target with said radiation, and movingsaid reference scatterers in order to scan the target.
 13. The method ofclaims 1, 2, 3, 4, 5, 6, 7, or 8 wherein the target is examined aplurality of times in succession, each examination comprisingcollimating said radiation to a beam of a different cross-section. 14.The method of claims 1, 2, 3, 4, 5, 6, 7, or 8 further comprisingimaging the density of the target by detecting the photons transmittedthrough the target.
 15. The method of claims 1, 2, 3, 4, 5, 6, 7, or 8further comprising imaging the density of the target by detecting thephotons back-scattered from the target.
 16. The method of claims 1, 2,3, 4, 5, 6, 7, or 8 further comprising ranging, wherein the location ofa nuclear species of interest in a target is approximately by employinga pulsed source of bremsstrahlung radiation and by measuring the delayof photons scattered from the target.
 17. A device for detectingexplosive materials in a target comprising:a shielded source ofbremsstrahlung radiation, means for making the bremsstrahlung radiationincident on the target, one or more reference resonance scattererspositioned such that bremsstrahlung radiation transmitted through thetarget will be incident on said reference scatterers, one or moredetecting apparatuses each comprising an array of individual detectorsfor capturing, measuring, counting, and recording the energies ofphotons scattered from said reference scatterers, and computingapparatus for accepting data from said detecting apparatuses, and forusing this data to determine if the target is likely to containexplosive materials.
 18. A device for detecting one or more nuclearspecies in a target comprising:a shielded source of bremsstrahlungradiation, means for making the bremsstrahlung radiation incident on thetarget, and one or more reference resonance scatterers positioned suchthat bremsstrahlung radiation transmitted through the target will beincident on said reference scatterers, one or more detecting apparatuseseach comprising an array of individual detectors for capturing,measuring, counting, and recording the energies of photons scatteredfrom said reference scatterers.
 19. The device of claim 17 furthercomprising detecting apparatus comprising an array of individualdetectors for capturing, measuring, counting, and recording the energiesof photons scattered from the target, said computing apparatus adaptedto accept data from said detecting apparatus, and for using this data todetermine if the target is likely to contain explosive materials. 20.The device of claim 18 further comprising detecting apparatus comprisingan array of individual detectors adapted to capture, measure, count, andrecord the energies of photons scattered from the target.
 21. The deviceof claims 17, 18, 19, or 20 wherein said source of bremsstrahlungradiation comprises an electron accelerator producing electrons incidenton a bremsstrahlung target.
 22. The device of claim 21 wherein saidsource of bremsstrahlung radiation further comprises a beam stopperfollowing said bremsstrahlung target for absorbing substantially allelectrons exiting said bremsstrahlung target.
 23. The device of claims17, 18, 19, or 20 wherein said source of bremsstrahlung radiationcomprises an aperture for collimating said bremsstrahlung radiation to abeam of a desired shape and angular spread.
 24. The device of claims 17,18, 19, or 20 wherein said source of bremsstrahlung radiation comprisesa filter for absorbing the low energy photons of the bremsstrahlungspectrum preferentially over the high energy photons.
 25. The device ofclaims 17 or 19 wherein said detecting apparatus counts scatteredphotons of energy about 4.92 MeV to detect nitrogen in the target. 26.The device of claims 17 or 19 wherein said detecting apparatus countsscattered photons of energy about 7.03 MeV to detect nitrogen in thetarget.
 27. The device of claims 17 or 19 wherein said detectingapparatus counts scattered photons of energy about 6.92 MeV to detectoxygen in the target.
 28. The device of claims 17 or 19 wherein saiddetecting apparatus counts scattered photons of energy about 7.12 MeV todetect oxygen in the target.
 29. The device of claims 17 or 19 whereinsaid detecting apparatus counts scattered photons of energy about 4.44MeV to detect carbon in the target.
 30. The device of claims 17 or 19wherein at least one of said reference scatterers comprises nitrogen.31. The device of claims 17 or 19 wherein at least one of said referencescatterers comprises oxygen.
 32. The device of claims 17 or 19 whereinat least one of said reference scatterers comprises carbon.
 33. Thedevice of claims 17, 18, 19, or 20 wherein said detecting apparatusesdetect photons scattered at an angle of greater than 90 degrees from thedirection of said bremsstrahlung beam.
 34. The device of claims 17, 18,19, or 20 wherein said detecting apparatuses detect photons scattered atan angle of approximately 150 degrees from the direction of saidbremsstrahlung beam.
 35. The device of claims 17, 18, 19, or 20 whereinsaid individual detectors are intrinsic germanium ionization chambers.36. The device of claims 17, 18, 19, or 20 wherein said individualdetectors are scintillators.
 37. The device of claims 17, 18, 19, or 20further comprising a filter in front of said individual detectors toabsorb the low energy scattered photons preferentially over the highenergy photons.
 38. The device of claims 17 or 19 wherein said means formaking said bremsstrahlung beam incident on the target is a conveyorbelt which moves the target into position in front of the aperture. 39.The device of claims 17, 18, 19, or 20 further comprising means formaking said individual detectors direction-specific.
 40. The device ofclaim 39 wherein said means for making said individual detectorsdirection-specific comprises a thickness of a high atomic numbershielding material over each individual detector with a column of saidshielding material removed, the axis of said column pointing in aspecific direction, whereby each individual detector will detect onlyphotons scattered from a particular location in a particular direction.41. The device of claim 17 or 19 wherein said computing apparatuscompares the data from the detecting apparatuses to profiles of datastored in said computing apparatus, said profiles corresponding tocomparable targets not containing explosives and/or to comparabletargets containing explosives to determine if the target is likely tocontain explosives.
 42. The device of claim 17 or 19 wherein saidcomputing apparatus compares the data from the detecting apparatuses toreference data corresponding to the signals received by the detectingapparatuses when the target is not placed in the path of thebremsstrahlung beam.